n(A)= 18 this item corrsponds to the number of observations inside A
n(B)= 11 this item corresponds to the number of observations inside B
n(A∪B)= 23 this item shows you the number of observations that are shared by A and B
n(A')=8 assuming A' is the complementary event of A, it indicates all observations inside the sample space that are outside A.
Total observations included in A are:
A= n(A) + n(A∪B)= 18 + 23= 41
Total of observations in B are:
B= n(B) + n(A∪B)= 11 + 23= 34
Total of observations in U
U= A + B + n(A') - n(A∪B)= 41 + 34 + 8 -23= 60
Note that I subtracted n(A∪B) one time, because is included in both A and B, i.e. is counted two times and could lead to a wrong answer.